Cop's wife feared he would eat her

CHARGED: Former New York City police officer Gilberto Valle, dubbed by local media as the "Cannibal Cop", appears in this courtroom sketch during opening arguments of his federal trial in New York.

A New York City police officer's bizarre internet banter about cannibalism wasn't harmless fantasy but a serious plot to abduct, torture and eat "very real women", a federal prosecutor said at the officer's kidnapping conspiracy trial.

"Make no mistake," Assistant US Attorney Randall Jackson told a jury on Monday (local time). "Gilberto Valle was very serious about these plans."

Valle, 28, a college graduate and father of a young child, appeared to be leading a normal life before he began acting "extremely strangely", Jackson said.

It became clear to Valle's wife that something was wrong, so she put a program on his computer to trace where he went online, Jackson said. Online discussions revealed his plans to "sexually assault these women ... to slit their throats and detailed plans to cannibalise these women", he said.

Valle's wife reported his behaviour to the FBI last year. The government was expected to call Valle's wife, Kathleen Mangan, as its first witness in one of the strangest cases in federal court in recent memory.

Claims by lawyers for Valle that he was only indulging in fetish fantasies are "utterly bogus", Jackson added.

The defence was to give its opening later Monday.

Valle is charged with conspiring to kidnap a woman and unauthorised use of a law enforcement database, which prosecutors say he used to help build a list of potential targets. A conviction on the kidnapping count carries a possible life sentence.

Prosecutors said in court papers that Mangan fled their home on September 10, 2012, believing her husband was planning to kidnap, rape, torture and murder her. She also discovered that he "had similarly heinous plans for women she knew and to whom she was close", prosecutors wrote.

The defence has denied that Mangan was a potential victim. Valle had made clear that his wife "was unavailable for any kidnapping fantasy", the defence papers said.

Valle, a baby-faced tabloid sensation, was expected to take the stand to make the case that it was all role-playing fantasy played out on Internet sites.

The defence planned to show jurors the videotaped testimony of the co-founder of one such site, Sergey Merenkov, in which he describes the site as a "social media network".

Asked the most popular fetishes, Merenkov responds, "all sorts of asphyxiation" and "peril cannibalism".

Tiger Howard Devore, a psychologist and certified sex therapist who specialises in dealing with sexual dysfunction and fetishes, said the cannibalism fetish known as voreaphilia isn't common.

"For most laymen, they're going to think about it as cannibalism," Devore said on Monday. "But what it really is, is an obsession about consuming the flesh of the other, and this can have a whole range of expressions. ... It is mostly played out in fantasy, mostly played out in role-playing."

There are well-known criminal extremes like Jeffrey Dahmer, who saved pieces of his victims' body parts and ate the flesh, Devore said, though "the instances of this kind of violence are extremely rare".